702 research outputs found
Galaxy Morphologies in the Cluster CL1358+62 at z=0.33
We describe the morphological composition of a sample of 518 galaxies in the
field of CL1358+62 at z=0.33, drawn from a large HST mosaic covering 53 sq.
arcmin. The sample is complete to I=22, corresponding to M_V=-18.5 in the rest
frame. The galaxies have been independently classified by the authors of this
paper and by Alan Dressler. For galaxies with I<21, the two sets of classifiers
agree on the total early-type population, but disagree on the S0/E ratio. We
discuss the constraints on morphological evolution and the implication of the
differing S0/E ratios. We use our large body of spectra to make the
correspondence between morphological and spectral type.Comment: includes 10 fig
Infall, the Butcher-Oemler Effect, and the Descendants of Blue Cluster Galaxies at z~0.6
Using wide-field HST/WFPC2 imaging and extensive Keck/LRIS spectroscopy, we
present a detailed study of the galaxy populations in MS2053--04, a massive,
X-ray luminous cluster at z=0.5866. Analysis of 149 confirmed cluster members
shows that MS2053 is composed of two structures that are gravitationally bound
to each other; their respective velocity dispersions are 865 km/s (113 members)
and 282 km/s (36 members). MS2053's total dynamical mass is 1.2x10^15 Msun.
MS2053 is a classic Butcher-Oemler cluster with a high fraction of blue members
(24%) and an even higher fraction of star-forming members (44%), as determined
from their [OII] emission. The number fraction of blue/star-forming galaxies is
much higher in the infalling structure than in the main cluster. This result is
the most direct evidence to date that the Butcher-Oemler effect is linked to
galaxy infall. In terms of their colors, luminosities, estimated internal
velocity dispersions, and [OII] equivalent widths, the infalling galaxies are
indistinguishable from the field population. MS2053's deficit of S0 galaxies
combined with its overabundance of blue spirals implies that many of these
late-types will evolve into S0 members. The properties of the blue cluster
members in both the main cluster and infalling structure indicate they will
evolve into low mass, L<L* galaxies with extended star formation histories like
that of low mass S0's in Coma. Our observations show that most of MS2053's blue
cluster members, and ultimately most of its low mass S0's, originate in the
field. Finally, we measure the redshift of the giant arc in MS2053 to be
z=3.1462; this object is one in only a small set of known strongly lensed
galaxies at z>3.Comment: Accepted by ApJ. Version with full resolution figures available at
http://www.exp-astro.phys.ethz.ch/tran/outgoing/ms2053.ps.g
The Evolution of Rest-Frame K-band Properties of Early-Type Galaxies from z=1 to the Present
We measure the evolution of the rest-frame K-band Fundamental Plane from z=1
to the present by using IRAC imaging of a sample of early-type galaxies in the
Chandra Deep Field-South at z~1 with accurately measured dynamical masses. We
find that evolves as , which is
slower than in the B-band (). In the B-band
the evolution has been demonstrated to be strongly mass dependent. In the
K-band we find a weaker trend: galaxies more massive than
evolve as ;
less massive galaxies evolve as . As
expected from stellar population models the evolution in is slower than
the evolution in . However, when we make a quantitative comparison, we
find that the single burst Bruzual-Charlot models do not fit the results well,
unless large dust opacities are allowed at z=1. Models with a flat IMF fit
better, Maraston models with a different treatment of AGB stars fit best. These
results show that the interpretation of rest-frame near-IR photometry is
severely hampered by model uncertainties and therefore that the determination
of galaxy masses from rest-frame near-IR photometry may be harder than was
thought before.Comment: 5 pages, 3 figures, Accepted for publication in ApJ
Mass-to-Light Ratios of Field Early-Type Galaxies at z~1 from Ultra-Deep Spectroscopy: Evidence for Mass-dependent Evolution
We present an analysis of the Fundamental Plane for a sample of 27 field
early-type galaxies in the redshift range 0.6<z<1.15. The galaxies in this
sample have high S/N spectra obtained at the VLT and high resolution imaging
from the ACS. We find that the mean evolution in M/L of our sample is , with a large galaxy-to-galaxy scatter. This value can
be too low by 0.3 due to selection effects, resulting in . The strong correlation between M/L and rest-frame color
indicates that the observed scatter is not due to measurement errors, but due
to intrinsic differences between the stellar populations of the galaxies. This
pace of evolution is much faster than the evolution of cluster galaxies.
However, we find that the measured M/L evolution strongly depends on galaxy
mass. For galaxies with masses , we find no significant
difference between the evolution of field and cluster galaxies: Delta ln (M/L_B) =
-1.12+/-0.06z$ for cluster galaxies. The relation between the measured M/L
evolution and mass is partially due to selection effects. However, even when
taking selection effects into account, we still find a relation between M/L
evolution and mass, which is most likely caused by a lower mean age and a
larger intrinsic scatter for low mass galaxies. Results from lensing early-type
galaxies, which are mass-selected, show a very similar trend with mass. This,
combined with our findings, provides evidence for down-sizing. Previous studies
of the rate of evolution of field early-type galaxies found a large range of
mutually exclusive values. We show that these differences are largely caused by
the differences between fitting methods. (Abridged)Comment: figures 3 and 4 available at
http://www.strw.leidenuniv.nl/~vdwel/private/FPpaper
The Extended IRTF Spectral Library: Expanded coverage in metallicity, temperature, and surface gravity
We present a spectral library of 284 stars observed with the
medium-resolution infrared spectrograph, SpeX, at the 3.0 meter NASA Infrared
Telescope Facility (IRTF) on Maunakea, Hawaii. This library extends the
metallicity range of the IRTF Cool Star library beyond solar metallicity to
[Fe/H] . All of the observed stars are also in the MILES
optical stellar library, providing continuous spectral coverage for each star
from . The spectra are absolute flux calibrated using Two Micron
All Sky Survey photometry and the continuum shape of the spectra is preserved
during the data reduction process. Synthesized colors agree with
observed colors at the level, on average. We also present a spectral
interpolator that uses the library to create a data-driven model of spectra as
a function of , , and [Fe/H]. We use the library and interpolator
to compare empirical trends with theoretical predictions of spectral feature
behavior as a function of stellar parameters. These comparisons extend to the
previously difficult to access low-metallicity and cool dwarf regimes, as well
as the previously poorly sampled super-solar metallicity regime. The library
and interpolator are publicly available.Comment: Accepted to ApJS. The website making the data publicly available will
be available soon. For those interested in the meantime, contact the first
autho
Morphological Evolution and the Ages of Early-Type Galaxies in Clusters
Morphological and spectroscopic studies of high redshift clusters indicate
that a significant fraction of present-day early-type galaxies was transformed
from star forming galaxies at z<1. On the other hand, the slow luminosity
evolution of early-type galaxies and the low scatter in their color-magnitude
relation indicate a high formation redshift of their stars. In this paper we
construct models which reconcile these apparently contradictory lines of
evidence, and we quantify the effects of morphological evolution on the
observed photometric properties of early-type galaxies in distant clusters. We
show that in the case of strong morphological evolution the apparent luminosity
and color evolution of early-type galaxies are similar to that of a single age
stellar population formed at z=infinity, irrespective of the true star
formation history of the galaxies. Furthermore, the scatter in age, and hence
the scatter in color and luminosity, is approximately constant with redshift.
These results are consequences of the ``progenitor bias'': the progenitors of
the youngest low redshift early-type galaxies drop out of the sample at high
redshift. We construct models which reproduce the observed evolution of the
number fraction of early-type galaxies in rich clusters and their color and
luminosity evolution simultaneously. Our modelling indicates that approx. 50%
of early-type galaxies were transformed from other galaxy types at z<1, and
their progenitor galaxies may have had roughly constant star formation rates
prior to morphological transformation. After correcting the observed evolution
of the mean M/L_B ratio for the maximum progenitor bias we find that the mean
luminosity weighted formation redshift of stars in early-type galaxies
z_*=2.0^{+0.3}_{-0.2} for Omega_m=0.3 and Omega_Lambda=0.7. [ABRIDGED]Comment: Accepted for publication in The Astrophysical Journal. 13 pages, 6
figure
HST Photometry and Keck Spectroscopy of the Rich Cluster MS1054-03: Morphologies, Butcher-Oemler Effect and the Color-Magnitude Relation at z=0.83
We present a study of 81 I selected, spectroscopically-confirmed members of
the X-ray cluster MS1054-03 at z=0.83. Redshifts and spectral types were
determined from Keck spectroscopy. Morphologies and accurate colors were
determined from a large mosaic of HST WFPC2 images in F606W and F814W.
Early-type galaxies constitute only 44% of this galaxy population. Thirty-nine
percent are spiral galaxies, and 17% are mergers. The early-type galaxies
follow a tight and well-defined color-magnitude relation, with the exception of
a few outliers. The observed scatter is 0.029 +- 0.005 magnitudes in restframe
U-B. Most of the mergers lie close to the CM relation defined by the early-type
galaxies. They are bluer by only 0.07 +- 0.02 magnitudes, and the scatter in
their colors is 0.07 +- 0.04 magnitudes. Spiral galaxies in MS1054-03 exhibit a
large range in their colors. The bluest spiral galaxies are 0.7 magnitudes
bluer than the early-type galaxies, but the majority is within +- 0.2
magnitudes of the early-type galaxy sequence. The red colors of the mergers and
the majority of the spiral galaxies are reflected in the fairly low
Butcher-Oemler blue fraction of MS1054-03: f_B=0.22 +- 0.05. The slope and
scatter of the CM relation of early-type galaxies are roughly constant with
redshift, confirming previous studies that were based on ground-based color
measurements and very limited membership information. However, the scatter in
the combined sample of early-type galaxies and mergers is twice as high as the
scatter of the early-type galaxies alone. This is a direct demonstration of the
``progenitor bias'': high redshift early-type galaxies seem to form a
homogeneous, old population because the progenitors of the youngest present-day
early-type galaxies are not included in the sample.Comment: Accepted for publication in the ApJ. At
http://astro.caltech.edu/~pgd/cm1054/ color figures can be obtaine
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